There are several synonyms for the phrase "break one's promise." One of the most common alternatives is "renege," which means to go back on a promise or contract. Another synonym is "default," which is often used when referring to failure to fulfill a financial obligation. "Betray" is another synonym that implies a deeper level of deceit or disloyalty. "Fail to keep" is a more straightforward and simple phrase that can be used in lieu of "break one's promise." Regardless of the synonym used, the act of failing to honor a promise is often seen as a breach of trust and can result in damage to relationships and reputation.